The present invention is directed to a heat-shrinkable polyester film, more particularly, to a heat-shrinkable polyester film used for labeling or shrink-wrapping containers, which has improved antistatic and antiblocking properties.
Heat shrinkable films have been extensively used e.g., for labeling bottles, batteries or electrolytic condensers, and covering containers and other products. Such heat-shrinkable films are formed of polyvinyl chloride, polystyrene and polyester, and they are required to have good sealing and shrinking properties as well as good heat resistance, chemical resistance, weatherability and printability.
Conventional heat-shrinkable films formed of polyvinyl chloride or polystyrene have relatively poor heat resistance, chemical resistance, weather resistance and heat-shrinkable properties. In particular, polyvinyl chloride-based heat-shrinkable films recently become increasingly disfavored because they emit toxic pollutants when incinerated Polystyrene films, on the other hand, have the problem of poor printability, requiring a special purpose ink. Polystyrene films also tend to undergo marked spontaneous shrinkage during long-term storage.
Heat-shrinkable polyester films formed of polyethylene terephthalate (PET) have satisfactory heat resistance, chemical resistance, weatherability and shrinking properties. However, the shrinkage stress and shrinkage ratio of a PET film are generally unacceptably high, giving non-uniform shrink-wrappings with a consequential distorting of an image printed thereon.
Japanese Laid-open Patent Publication Nos. 63-139725, 7-53416, 7-53737, 7-216107, 7-216109 and 9-254257 teach that the shrinkage rate of PET film can be adjusted by blending with a polyethylene terephthalate or a polybutylene terephthalate in a predetermined ratio, or by copolymerizing a dicarbonic acid component such as terephthalic acid and isophthalic acid with a diol component such as ethylene glycol and 1,4-cyclohexanedimethanol. Such heat-shrinkable films show some improvements in terms of uniform shrinkage, but they tend to accumulate electrostatic charges on the surface during handling and processing. The electrostatic charge attracts not only dust and other contaminants but also other film, causing ink blur, blocking, wrinkles and bad appearances.
To deal with this electrostatic problem of polyester films, U.S. Pat. No. 4,632,263 discloses a polyester film coated with a composition comprising a polymeric binder such as (meth)acrylic polymer, vinyl acetate polymer, vinylidene chloride polymer, and a quaternary ammonium salt, and U.S. Pat. No. 4,214,035, a biaxially oriented polyester film having a latex coating containing a crosslinkable acrylate polymer and ammonium nitrate. However, when the antistatic coatings taught in these patents are applied to a heat shrinkable film, the coating layer does not firmly adhere to the film surface and also shows poor water resistance. Therefore, such coating layer may be easily washed off in a moist environment with consequential loss of antistatic properties.
Japanese Laid-open Publication No. 5-271523 discloses a coating comprising a water-soluble polyester, a conductive polymer and a surfactant. However, such a composition containing a surfactant may become foamy during a coating process, besides the problem of its poor water-resistance.
Accordingly, it is a primary object of the present invention to provide a heat shrinkable film having improved surface properties including antistatic properties, antiblocking, printability, slipping properties and thermal sticking resistance as well as thermal shrinkage properties.
The above object of the invention can be accomplished by a heat-shrinkable polyester film comprising a polyester film support and a coat layer formed on one or both sides of the polyester film support, said coat layer comprising a quaternary ammonium sulfate of formula (I) and a polysiloxane resin of formula (II): 
wherein,
R is CH3, OH, H or COOH;
Rxe2x80x2 is CH3, COOH, OH or CH2CH2OH;
X is Cl, OH, OCH3, OC2H5, H, NH2, N(CH3)2 or CHxe2x95x90CH2; and
n is a number in the range of 2 to 15.